Inkjet printing technology is well known for use in printing images onto paper. Inkjet technology is also used in the fabrication of printed circuits by directly printing circuit components onto circuit substrates. Inkjet printing-based approaches for high resolution manufacturing have inherent advantages and are of interest for a number of reasons. First, functional inks are deposited only where needed, and different functional inks are readily printed to a single substrate. Second, inkjet printing provides the ability to directly pattern wide classes of materials, ranging from fragile organics or biological materials that are incompatible with other established patterning methods such as photolithography. Third, inkjet printing is extremely flexible and versatile in that structure design changes are easily accommodated through software-based printing control systems. Fourth, inkjet printing is compatible with printing on large area substrates. Finally, inkjet systems are relatively low cost and have low operating cost. Such advantages are one reason why inkjet printing technology is used in a number of applications in electronics, information display, drug discovery, micromechanical devices and other areas.
Two common methods for jetting fluid from printheads are drop-on-demand and continuous inkjet. Two types of drop-on-demand ink jet printers that are commercially successful use thermal or piezoelectric means for ink printing. In both types, the liquid ink is transferred from a reservoir to paper substrate by applying a pressure to the reservoir, and printing occurs in an all-or-none fashion. In other words, they either print a dot at a fixed size when the reservoir pressure is above a threshold level, or do not print at all when the reservoir pressure is below a threshold level. The functional resolution of these conventional systems is limited to about 20 μm to 30 μm. A third class of inkjet printing systems is known as electrohydrodynamic printing.
Electrohydrodynamic jet (e-jet) printing is different from the inkjet printers that rely on thermal or piezoelectric pressure generating means. E-jet printing uses electric fields, rather than the traditional thermal or acoustic-based ink jet systems, to create fluid flows to deliver ink to a substrate (e.g., see U.S. Pat. Nos. 5,838,349; 5,790,151). E-jet systems known in the art are generally limited to providing droplets having diameter greater than 15 μm using nozzle diameters that are greater than 50 μm. The general set-up for e-jet printing involves establishing an electric field between a nozzle containing ink and the paper to which the ink is transferred. This can be accomplished by connecting each of a platen and the nozzle to a voltage power supply, and resting electrically conductive paper against the platen. A voltage pulse is created between the platen and the nozzle, creating a distribution of electrical charge on the ink. At a voltage pulse that exceeds a threshold voltage, the electric field causes a jet of ink to flow from the nozzle onto the paper, either in the form of a continuous ink stream or a sequence of discrete droplets.
E-jet processes are generally linear, unlike the thermal or piezoelectric processes, in that the amount of ink transferred is proportional to the amplitude and duration of the voltage difference. Accordingly, e-jet printing offers the capability of modulating the size of individual dots or pixels to generate high-quality images of comparable quality to expensive dye diffusion printers. U.S. Pat. No. 5,838,349 recognizes the difficulty of e-jet printing onto insulating materials and multiple color printing onto a single surface by improper registration (caused by charge retainment of printed ink affecting nearby subsequent printing), and proposes overcoming registration issues by providing a means to ensure uniform charge on the substrate surface to be printed. In that system, the printing nozzle is about 0.5 to 1.0 mm from the platen with an inside nozzle diameter ranging from 0.1 mm to 0.3 mm.
Typically, in the graphical arts applications e-jet printing involves printing inks that are pigments from a nozzle having a diameter of about 40 μm or greater to generate a printed dot diameter that is at best, about 20 μm or greater. Typically, the voltage is about 1.5 kV at a stand-off distance of about 500 μm. In manufacturing applications, inks are often metal and SiO2 nanoparticles, cells, CNTs (carbon nanotubes), etc that are printed from a nozzle having a diameter about 50 μm or greater, generating a printed line having a width that is at best about 20 μm or greater. Similarly, the voltage is about 1.5 kV with a stand-off distance of about 300 μm or greater. See, e.g., Appl Phys Lett. 90 081905 (2007), 88, 154104 (2006); Lab Chip. 6, 1086 (2006); Chem. Eng. Sci. 61, 3091 (2006); Guld Bull. 39, 48 (2006); J. Nano. Res. 7, 301 (2005); J. Imaging Sci. 49, 19 (2005); IS&Ts NIP. 15, 319 (1999) and 14, 36 (1998); Recent Progress in Inkjet II. 286 (1999); IBM Report. RJ8311, 75672 (1991). Because of potential adverse effects such as nozzle clogging, it is believed that there are disadvantages to decreasing nozzle diameter less than about 30 μm. For example, in many ink jet printing applications using electrohydrodynamic-generated printing, the nozzle diameter from which ink is ejected is on the order of 0.0065 inches (165 μm) (See, e.g., U.S. Pat. No. 5,790,151)
In a number of applications, lines or smallest gaps that can be reliably created is about 20 to 30 μm. This resolution limit is due to the combined effects of droplet diameters that are usually no smaller than about 10 to 20 μm (corresponding to 2-10 pL) and placement errors that are typically plus or minus about 10 μm at standoff distances of about 1 mm. Through the use of separate patterning systems and processing steps, the resolution may be decreased to the sub-micron level. For example, lithographic processing of the substrate surface that is to be printed may assist in localizing features into certain preferred locations. The ink that is being printed may be surface functionalized prior to printing. The substrate may be processed in patterns of hydrophobicity or wettability, or have relief features for confining and guiding the flow of droplets as they land on the substrate surface. Accordingly, printed features may achieve, when combined with one or more of such processing features, sub-micron resolution. Those additional steps, however, do not provide a general approach to achieving high resolution in that they must be tailored for each printing system. Furthermore, they require separate patterning systems and processing systems adding to manufacturing expense and time.
Accordingly, there is a need in the art for e-jet systems capable of providing high-resolution patterning and for fabricating devices in a range of applications (e.g., electronics) by using functional or sacrificial inks.